A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In optical projection lithography protection against particulate contamination of a patterning device surface carrying (or arranged to form) a pattern to be projected onto the substrate is typically realized by providing that surface with a pellicle. A pellicle generally comprises a pellicle membrane (a sheet or foil) transmissive for radiation used for exposure and a frame carrying the pellicle membrane. The frame carrying the pellicle membrane is bonded to, for example, a reticle surface so that the membrane is disposed at a distance from the patterned surface. In the presence of the pellicle, particulate contaminants cannot reach that surface, and hence stay out of focus during use of the reticle in the apparatus. The pellicle membrane can, for example, be a polymeric membrane or a fused silica membrane. Pellicles of the latter type are commonly referred to as “hard” pellicles. A pellicle membrane (a polymeric membrane or a hard pellicle membrane) acts as a refractive optical element on the projection beam, and therefore control of the shape of the refracting surfaces of the membrane is critical. For example, a non flat pellicle membrane shape may cause out of tolerance image errors of an image of the pattern, as projected onto the substrate. These image errors comprise optical aberrations such as, for example, distortion and defocus. Attachment of a pellicle to a reticle may also cause deformations of the reticle, which in turn cause similar image errors.
Pellicles are manufactured separate from lithographic apparatus, in accordance with specifications concerning maximum allowable deviations from a nominal shape of the pellicle membrane surface. Control of surface shape of the pellicle membrane is particularly critical with the use of hard pellicles, because generally hard pellicles are thicker than polymeric membranes, and may therefore act more strongly as a refractive optical element on the projection beam than the relatively thin polymeric membranes. For increasing Numerical Aperture (NA) of the projection system of the apparatus, the control of shape of the pellicle membrane becomes even more critical, and there is the problem of complying with pellicle-shape specifications not only after manufacture of a pellicle but also during use of the pellicle combined with a reticle, in a lithographic apparatus, while maintaining any reticle deformation, due to attachment of a pellicle to the reticle, within tolerance.